Accelerometer with electric strain wires



Nov. 9, 1948. L. D. srA'rHAM ACCELEROKETER WITH ELECTRIC STRAIN WIRES Filed July 14, 1947 I NvEN 701.

' i D. Sini/mm Patented Nov. 9, 1948 ACCELEROMETER WITH ELECTRIC STRAIN WIRES Louis D. Statham, Beverly Hills, Calif., assigner to Statham Laboratories, Inc., Los Angeles, Calif., a corporation of California Application July 14, 1947,`Serlal No. 760,775

Claims.

This invention relates to a strain wire accelerometer. Strain wire accelerometers employing unbonded strain wires have found great utility, and such accelerometers are described in an article by Richard D. Meyer, published in Instruments, vol. 19, No. 3, of March, 1946.

Accelerometers employing strain wires such as are described in the above article are of two types: 'I'he beam type comprising weighted cantilever beams to which a bonded strain gauge is attached to measure the bending of the beam. The strain wires do not .enter into the system as a force transmitting or resisting element. The other type may be termed a suspended mass accelerometer in which a mass is supported on the strain wires of a strain gauge. The forces of acceleration are not applied to a comparatively stiff beam, but directly to the strain sensitive wires.

Essentially such accelerometers are composed of a frame upon which a mass is suspended by Wire springs so that the mass moves upon acceleration in adirection substantially parallel to the wires. In one form the mass carries four pins which coact with four pins on the frame.' Four loops of strain sensitive wire are wound on the pins. These wires are wound under tension. Two loops extend in one direction from the axis of the mass to the frame and two loops extend in an opposite direction. Upon movement of the mass, which is constrained by springs to move in a path parallel to the strain wires, the loops on one side of the vertical axis are shortened to release the strain, while the wires on the two loops are lengthened to increase the strain. 'I'he permissible travel of the mass is controlled by `a stop an'd is limited not to exceed about 0.0015 eithery side of neutral.

Another form which has been suggested suspends the mass on three loops of wires, all ending in a direction perpendicular each to the other and the mass being free to move in either a horizontal or vertical plane. The wires thus extend in a direction parallel to the direction of each of the three degrees of freedom of motion of the suspended mass. y

In all such strain wire assemblies where the function depends upon the changein resistance of the strain wires,"in order to get measurable changes in resistance it is necessary to use wires of considerable length, and therefore it is necessary to separate the points of support of the wires by a considerable distance, and the gauge in the direction of the applied accelerating force or in the direction of motion of the accelerated mass will necessarily be of substantial dimension.

For many uses the space requirements make it highly advantageous to reduce the dimension of the instrument in the direction of the accelerating force. Thus, in measuring the acceleration of the airplane wing tips, where the accelerometer is to be placed inside the wing, it is essential to place the accelerometer at the tip where space restrictions are limited.

I have devised an unbonded strain wire vetc-- celerometer in which the strain wires are all disposed in a direction substantially perpendicular to the line of the accelerating force and substantially perpendicular to the permissible motion of the accelerated mass, and thus have been able to reduce the dimension of this instrument in this direction without sacrificing the output or sensitivity of the instrument.

In linear accelerometers employing unbonded strain wire elements, in which the accelerated mass is suspended directly on the strain wires. the accelerating force is transmitted directly to the wires. Thus, to obtain a practical power output from the instrument, that is, an appreciable total change in the resistance of the strain wires of the instrument, it is necessary to have wires of appreciable length and thus the weight which is required may become large. This increases thev dimension and weight of the instrument.

However, in many applications, especially where the acceleration of a small mass is to be measured, it is desirable to hold the mass of the instrument as small as possible. Thus it is desirable that the mass of the instrument does not add appreclably to the mass Whose acceleration is to be measured.

Y It thus becomes desirable to reduce the mass of the accelerometer. It is desirable to do so, however, without appreciably reducing the power output of the accelerometer and this presents a dilemma, since in the accelerometers where the mass is directly suspended or connected to the strain wires, such as those mentioned above, these are contradictory requirements.

I have devised a. new form of linear accelerometer which solves theabove dilemma.l and have thus been able to produce a linear accelerometer of a reduced mass without sacrificing power output.

I accomplish' this desirable result by building a mechanical advantage into the accelerometer whereby the inertial mass of the accelerometer is connected to the strain wires through a lever system so that the inertial mass acts upon the strain wires through the mechanical advantage or said lever system.

I thlm suspend the inertial mass upon a cantilever suspension about a hinge connected to the inertial mass at one end thereof and the other end being free, the hinge being also mounted on a frame or support. The inertial mass is restrained by means of strain Wires which extend to connect the inertial mass to the frame, so that when the inertial mass moves about its hinge point, the tension in the wires is altered to give a measurable variation in resistance.

In the preferred embodiment or my invention the wires are strung between the frame and the inertial mass so that the locus of motion of the center of mass of the inertial mass moves in a path substantially transverse-that is, perpendic-l ular to the wires.

The wires are strung between pins which are so positioned that one oi the pins is stationary in the frame while the other pin is mounted on the inertial mass near the hinge axis. When the inertial mass is deilected from rest, the pin on the inertial mass pivots through an arc which, by reason of the limit motion stops and the other parameters of the instrument, is less than about l".

ln this sense the locus oi motion or the center of mass may be taken to be substantially the tangent of the are o such locus and sensibly perpendicular to the strain wires.

The center of mass thus acts upon the strain wires through a lever system in the nature of a bell crank and thus acts through a mechanical advantage to multiply the force exerted on the strain wires by such lever ratio.

it will thus be seen that a smaller :mass is required to cause the same variation. in. tension in the wires than is required to canse such variation in tension when the inertial mass is suspended directly upon the wires.

Another desirable property of my acceleronieter is that the dimension of the instrument in the direction of the accelerating `torce is made small, since the wires whose length aidects the maximum dimension of the instrument extend in a direction perpendicular to the direction of such force.

A third property oi the alcove design is that for any given magnitude of accelerating torce the movement of the inertial mass is greater, for an equal total resistance change in the wires, than where the mass is suspended directly on the wires. Thus, natural frequency of my new accelerorneter is lower in the case or such other type. For some purposes this lower natural frequency may he advantageous.

lhese and other objects advantages will he clear 'from the further description oi my new accelerometer when read in connection with the drawings, in which Fig. l is a` horizontal section of my accelerometer tahen online i-i or Fig. 2;

2 is a vertical section taken on line Z--l oi? Fie. l;

Fig. 3 is a sectiontalsen on line 3-3 of Fig. l;

Fig. 4 is a plan view of the frame;

Fig. 5 is a section on line of Fie'. 4;

Fig. d is an exploded view of the spring hinge and clamping members.

The accelerometer is mounted inside of the case i which is covered by a cover` 2 set into a shoulder il formed in the bottom of case l. Case i carries a rim t cooperating with the rim d of the case i to form a base for the instrument. Upon the base is set a iiexible diaphragm ii which' is held in place by a case plate t having an by means of screws 8'.

upstanding rim l and which is clamped to case I There is thus provided chambers 'I0 and II on each side of the diaphragm 9. The chamber I2 inside the cover 2 and the space I I are connected by bore I3.

Mounted on posts I4 which clamp the accelerometer between the top of the case I and the cover 2 is the frame I5 of the accelerometer. Frame I5 is in the form of a U-shaped member having legs I6 and I1 and a base I8. The base I8 is slotted at I8' to give lips I9 and 20. The ends of the legs I6 and i-I are notched at 2l and 2i to give lips 22 and 22'. Positioned in lips I9 are four bores 23a, 23h, 23c, and 23d. Similarly positioned in lip 2D are iour bores 23'a, 23'12, 230, and 'd, axially aligned with the bores 23a to 23d, respectively. Also positioned in lip id are two bores 26a and 2Gb whose line of center is parallel to the line of center of the bores 23a to 23d and which are spaced intermediate the legs i3 -and il. Positioned in lip 2B are two bores 25'a and 25'b whose axes are coincident with the axes 24a and 2th respectively.

The inertial mass ZE is in the form of a block, one end of which is formed in two steps 2l and 2t. A flat iiexible spring 29 is clamped on the tread of step il by means oi an L-shaped clamping bloclr di which is held by means of two screws 32 which pass through bores provided in the spring and in the tread of the step 2l and in the Ils-shaped block iii. A spring 2i) is clamped to the frame by means of clamping bars 33 and dal. The long clamping har Si@ is held upon the lip l2 and ZZ by means oi screws passing through bores 3l, and ill. The spring 2li is clamped to the bar 3d by means of a clamping bar 3Q' and screws which pass through the hores ll, ill, and

respectively. Thus, these clearances are provided between the contiguous faces oi' the tread of the step 28, the riser or the step and the i.. races of the block. ill, and the opposing aces ci the clamping blocks 3i. and Sil, as shown in 2. The opposite end of the inertial mass E2G is freely suspended and is bored at the pin 2t' which passes through a slot 28a into which slot pass two adjusting stop screws lila, which pass through suitable bores provided in the lips i9 and 2d.

Positioned in the bores 2da and -flb are two anodized aluminum pins and positioned in the bores 25e and 25h on the underneath face ci the lip are two similar anodized pins Sila and Two anodized aluminum pins lido, and 3% are mounted on the upper face oi the inertial mass 2S in line with the pins 38a and Silo and the axes oi said pins lila and ich are in line with the hinged point oi the inertia mass, to wit, they are over the space between the clamping "oars and the rise oi the tread il@ and the opposing iace of the l.shaped block 3i. Two anodized aluminum pins dict and lib are positioned on the iniderneath iace ci the L-shaped block 3i coaxiaily disposed with the pins fida and ich and are therefore in line with the pins 39a, and 39h. Pins are thus positioned on each oi the opposite complementary coplanar faces of the inertial mass and o the frame, the pins on the opposite faces of the frame being positioned adjacent the free end of the inertial mass, and the pins on the opposite faces of the inertial mass complementary to the faces oi the frame being positioned on the inertial rnass adjacent the hinge axis,

Wound between the pins 33h and 40h is a loop of strain wire @2b similar to those described in the above mentioned article, and similarly wound on pins 38a and loa is a loop of similar strain vvire 42a. In like manner loops of wire 40a and 43h are wound on the pinsv lla and 39a and IIb and fb, respectively. The terminal posts in the form of metallic pins Ma, Mb, "c, and 44d, and Ma, M'b, 'c, and 'd, respectively pass through insulating bushings positioned in the bores 23a to 23d and 23' to 23'd respectively. The. ends of the loops 42h are connected respectively to the posts c and d as are the ends of the loops 43a connected to the posts "a vand Mb, respectively, and the ends of the loops 42'a are connected to the posts 'a and 'b, respectively, and the ends of the loops 42'?) are connected to the terminal posts 'c and M'd, respectively. The terminals 46a and 4Gb are suitably insulated therefrom by bushings.

It will be observed that because of the anodized pins the strain Wires are insulated from the frame and from the inertial mass and from the case, and

are electrically connected to the terminals 44a and 44d, as stated above. The wires are spaced from and are substantially parallel to the complementary faces of the inertial mass and frame, and thus constitute unbonded strain Wire elements. The pins 44a and Ma may be in the form of a single pin, as illustrated in the drawing, and in like manner pins Md and 'd may form one continuous metallic pin. The ends of the loop of the strain wire 42a are connected to the pins a and Mb and the ends of the loops 42h are connected to the pins c and the pins 44d. The ends of the loops 42'a are connected to the pins Ma and Mb and the ends of the loops l2'b are connected to the pins Mc and M'd. The terminal pin lia is connected to the pin Mc by an insulated cross wire connection and the terminal pin 41h is connected to the pin Mb. The pins c and M'b are cross connected by wire, making electrical contact with said pins and insulated from the frame. Terminal contact 41a connects the pins a and Ma. Terminal Alb connects to the pin Md and M'd. Terminal 41e connects to 'b and the terminal 46d connects to either 46a or 48h. The loops of wire are thus connected in a four-leg Wheatstone bridge arrangement with the balancing wire strung between the terminals 46a and 4Gb, acting as a balancing resistance to balance the Wheatstone bridge when the accelerometer is in rest position.

When such an accelerometer is mounted upon a base such as 48 it will measure acceleration normal to the base, i. e., inthe direction of arrow shown in Fig. 2.' The linear acceleration measured is in a direction substantially perpendicula'r to the wires. The dimension of the instrument in the direction of acceleration to be measured may then be made small without sacriiicing the power output; in other words, the wires may be made as long as is desirable Without increasing the height of the instrument in the direction of the accelerating force.

On acceleration of the instrument the inertial mass is deflected either side of the rest position shown in Fig. 2, depending upon the direction oi' the acceleration. Thus, if the instrument is accelerated, causing the pins a and 4Gb and la and llb to tilt from their normally vertical position, the length of wires 42a and 42h or 43a and It will thus be seen that the force imposed upon the wires and therefore the degree of extension or contraction of the wires upon any 1inear acceleration of the instrument normal to the strain wires is directly proportional to the mass of the inertial mass 28 times the distance of the center of mass from the hinge axis of the instrument, to wit, from the point of bending oi the spring 29. The accelerometer is thus linear in character. The main restoring force is the tension of the wires, the spring entering therein in an insubstantial manner, for example, contributing about 5 to 10% of the restraint.

The weight of the inertial mass that would be then necessary to obtain any given variation in the restraint of the wires is reduced by the mechanical advantage of this lever system. It has been found, for example, that it would be possible to use an inertial mass of weight from 1/3 to 1/5 of that which is required in the instruments according to the design described in the above mentioned article to obtain an equal power output, i. e., total variation in resistance of the strain wire, by reason of the fact that from 3 to 5 times the mechanical advantage which is built into the instrument according to the present design.

The same mechanical advantages require that the degree of deflection of the inertial mass from rest position for any given variation in the resistance of the Wires is greater than if the same inertial mass were suspended directly on the same strain wire as in the aforementioned design. The result is that the natural frequency of the instrument is lower by reason of the greater displacement upon the imposition of any given accelerating force which gives a like total power output.

While I have described a particular embodi ment of my invention for the purpose of illustra-v tion, it should be understood that various modi-1 ilcations andA adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A strain wire accelerometer, comprising a frame, an inertial mass hingedly mounted at one end upon said frame, the other end of said mass being free to move about said hinge, and a strain wire connected to said frame and said mass.

2. A strain wire accelerometer, comprising a frame having opposite faces, an inertial mass having opposite faces, a hinge mounted upon said frame and upon one end of said inertial mass, a strain wire support mounted on one face of said frame and another support mounted on one of the faces of said mass, a strain wire mechanically connected to said' supports, a strain wire support on the opposite face of said frame, a strain wire support on the opposite face of said mass, and a strain wire mechanically mounted on said last-named supports.

3. A strain wire accelerometer, comprising a frame having opposite faces, an inertial mass having opposite faces, a hinge mounted upon v said frame and upon one end of said inertial mass, a strain wire support mounted on one face of said frame and another support mounted on one of the faces of said mass, a strain wire mechanically connected to said supports, a strain wire support on the opposite face of said frame, a strain wire support on the opposite face of said mass, and a strain wire mechanically mounted on said last-named supports, said wires extending substantially parallel to said faces and substantially perpendicular to the axis of said hinge.

4. A strain wire acceleronieter. comprising a frame having opposite faces, an inertial mass having opposite faces complementary to the faces of said frame, a hinge mounted upon said frame and upon one end of said inertial mass, the other end of said mass being free, a pin mounted upon each of the opposite faces of said mass and positioned on said mass adjacent the airis of said hinge, pins mounted on the opposite iZaces oi said frame adjacent the iree end of said mass, and strainA wires mechanically mounted upon and extending between the pins on the compie mentary faces of said frame and mass.

5. n. strain wire accelerometer, comprising a traine having opposite faces, `an inertial mass having opposite faces complementary to the races of said frame, a hinge mounted upon said frame and upon one end ci said inertial mass, the other end o said mass being free, a pin mounted upon each of the opposite faces o said mass and positioned on said mass adjacent the axis of said hinge, pins mounted on the opposite faces of said frame adjacent the free end of said mass, and strain wires mechanically mounted upon and extending between the pins on the complementary strain wire extending in a direction perpendicular to the axis of bending of said spring. A

7. A strain wire accelerometer, comprising a frame having opposite faces, an inertial mass having opposite faces, a at spring, means for connecting said hinge to said frame and to one end of said inertial mass, the other' end of said inertial mass being free, a strain wire support mounted on one face of said frame and another supportmounted at one of the faces of said mass, a strain wire mechanically connected to said supports, a strain wire support on the opposite face of said frame, a strain wire support on the opposite face of said mass, and a strain wire mechanically mounted on said last-named supports.

8. A strain wire accelerometer, comprising a framehaving opposite faces, an inertial mass having opposite faces, a fiat spring, means for connecting said hinge to said frame and to one end of said inertial mass, the other` end of said inertial mass being free, a strain wire support mounted on one face of said frame and another support mounted at one of the faces of said mass., a strain Wire mechanicaily connected to said supports, a strain Wire support on the opposite face of said frame, a strain wire support on the oppom site face of said mass, and a strain Wiremechaui cally mounted on said last-named supports, said wires extending substantially parallel to said races and substantially perpendicular to the airis of bending of said spring.

9. .a strain wire acceiercrneter, comprising a frame having opposite faces, an inertial mass having opposite faces complementary to the faces of said frame, a fiat spring mounted on said frame and. upon one end of said inertial mass, the other end oi' said mass beinff free, pins mounted on each o the opposite faces of said mass and positioned on said mass adjacent the axis of bending oi said spring, pins mounted on the opposite faces of said frame adjacent the free end of said mass, and strain wires mechanically mounted on extending between ,the pins on the complemen tary faces of said frame and mass.

l0. A strain Wire accelerometer, comprising a frame having opposite faces, an inertial rnass having opposite faces complementary to the faces of said frame, a flat spring mounted on said frame and upon one end of said inertial mass, the other end of said mass being free, pins mounted on each of the opposite faces of said mass and positioned on said mass adjacent the axis of bending of said spring, pins mounted on the opposite faces of said frame adjacent the free end o said mass, strain wires mechanically mounted on and extending between the pins on the cornplenienn tary faces of said frame and mass and substantially parallel to the said faces and substantially perpendicular to the axis of bending oi said spring.

No references cited. 

